1. Field of the Invention
The present invention relates generally to video cameras, and more particularly to shutter mechanisms for video cameras. More specifically, the invention relates to improved focal plane shutters for producing stop action or slow motion pictures without blurring, skewing or distortion.
2. Description of the Prior Art
The capturing of physical images through video recording involves a process whereby light reflecting or emanating from objects within a scene is collected and converted into electrical energy, and then magnetically stored for replay at a later time. The typical video system includes an optical system comprising one or more high quality, color-corrected lenses for focusing an image on the photosensitive surface of a video pickup device. Optical focusing is achieved by moving the lense with respect to the pickup device or by moving the pickup device with respect to the lense. The light which reaches the photosensitive surface of the pickup tube or other pickup device represents the image of the scene being recorded.
The exposure or quantity of light reaching the pickup tube surface may be controlled by varying the exposure time, by varying the size of the lense aperture or opening through which the exposure is made, or by varying both exposure time and lense aperture size--all of which are related to one another through the principle of reciprocity. The principle of reciprocity states generally that exposure time and lense aperture size are inversely related and that an exposure of a given quantity of light can be achieved by a wide variety of exposure time/aperture size combinations. To vary the exposure time, rotary focal plane shutters are sometimes placed between the lense and the pickup tube or device. Prior art rotary focal plane shutters are discussed more fully below. To vary the lense aperture size, it is common to provide the optical system or lense with an iris diaphragm mechanism which comprises thin overlapping metal plates that can be adjusted to form an aperture of varying size. Such mechanisms are frequently calibrated in "f-stops".
Once the optical image reaches the video pickup it is converted into an electrical video signal. The pickup is used to generate a train of electrical pulses representing the light intensities present in the optical image which is or has been focused on the surface of the pickup. Each point or pixel of this image is interrogated in its proper turn by the pickup, and an electrical impulse corresponding to the amount of light at that point is generated. Usually the electrical impulses from a plurality of points are serially combined or concatenated to comprise the video signal. In many pickup devices popular today, each point is interrogated by an electron beam which is electrostatically or magnetically deflected back and forth across a prescribed pattern (called a raster) on the glass target found on the inside of the pickup device. Electronic deflection circuitry generates electrical waveforms which, when applied to deflection coils or the like, produce a linear scanning motion of the electron beam following the prescribed raster pattern. To improve the image it is common to generate an interlaced raster, whereby all odd numbered lines are first sequentially interrogated, followed by a vertical retrace of the beam to its point of origin, and then all even numbered lines are sequentially interrogated. During vertical retrace the electron beam is swept, usually diagonally, from its termination point at the end of the last line in the raster pattern or field to its origin point. During vertical retrace a blanking signal is generated for a duration or blanking interval sufficient to allow the beam to sweep from termination point to origin point. The blanking signal causes the pickup device to momentarily cut off its signal output so that retrace lines are not visible when the image is viewed or replayed.
Most present day video pickups comprise electron tubes although there are also solid state devices. Such electron tubes may be classified based on the method of signal generation. In a non-storage tube, the only light utilized in generating a signal is that light reaching a particular point on the tube's light sensitive region while that point is being scanned or interrogated. In a storage tube, on the other hand, an electric charge accumulates on the tube's light sensitive region at each point during the interval between successive scans for later interrogation. Because the storage-type tube uses the electric charges generated by the light during the comparatively long intervals between successive scans of the image, it is more efficient and more sensitive. Storage-type tubes are further classified according to whether the light sensitive element is photoemissive or photoconductive. When photoemissive materials absorb light they emit electrons. When photoconductive materials absorb light their electrical conductivity changes.
The "vidicon" tube is one such photoconductive storage tube which has gained great popularity due to its small size and simplicity of operation. The vidicon is a storage-type tube in which the signal output is developed directly from the target of the tube and is generated by a low velocity scanning beam from an electron gun. The target consists of a transparent signal electrode deposited on the face plate of the tube and a thin layer of photoconductive material, which is deposited over the electrode. The photoconductive layer serves two purposes. It is the light sensitive element, and it also forms the storage surface for the electrical charge pattern that corresponds to the light image falling on the signal electrode. The photoconductive material has a fairly high resistance when in the dark. Light falling on the material excites additional electrons into a conducting state, lowering the resistance of the photoconductive material at the point of illumination. In operation, a positive voltage is applied to one side of the photoconductive layer via the signal electrode. On the other side of the layer the scanning electron beam deposits low velocity electrons in sufficient numbers to maintain a net zero voltage. In the interval between successive scans of a particular spot, the incident light lowers the resistance in relation to its intensity. With the resistance lowered, current flows through the surface of the photoconductive layer and a positive charge is built up on the back surface of the layer; that positive charge is then held until the beam returns to scan the point. When the beam returns, a signal output current is generated at the signal electrode as this positively charged area returns to zero voltage.
In order to provide low velocity electrons in a uniform manner, a fine mesh screen is stretched across the interior of the tube near the target. The screen is energized to cause the electron scanning beam to decelerate uniformly at all points and to approach the target in a perpendicular manner. The beam is brought into sharp focus on the target by longitudinal magnetic fields produced by focusing coils surrounding the tube. The beam is made to scan the target in its characteristic raster pattern by varying magnetic fields produced by horizontal and vertical deflection coils also disposed about the tube.
Video systems of the prior art, including those using storage tube devices, have been historically plagued with difficult and troublesome blurring, skewing and distortion when used to produce slow motion or stop-action images. Slow motion and stop action images are created during playback by utilizing specially equipped video tape decks. Such tape decks, however, can only produce images as clear and sharp as the video camera which produced them. If the video camera produces blurred, fuzzy or distorted images, then the taped image will also be blurred, fuzzy and distorted. These undesirable effects are most apparent during slow motion or stop-action replay.
A prior art solution to the problem of blurred images is to interpose a rotary shutter between the lense and pickup of an otherwise standard video camera. The rotary shutter of the prior art comprises a circular disk provided with at least one matched pair of apertures spaced 180.degree. apart about the circumference. The disk is rotated at a constant speed in use (typically 1800 rpm). The apertures are positioned about a locus between lenses and pickup so that for two brief intervals per disk revolution, light images will illuminate the pickup tube. The apertures are pie-shaped openings or segments bounded by radial lines emanating from the center of rotation of the disk. Thus, by virtue of the fact that the pie-shaped apertures are paired and spaced 180.degree. apart, 3,600 individual or discrete exposures are made each minute at a rotational rate of 1800 rpm. The exposures are timed to occur during the blanking intervals, hence each full raster interrogation produces a train of electrical impulses which represents the image exposed during a previous blanking interval. By using very short time intervals a fast moving object, such as a rocket sled, or an athletic event, can be captured, stored and reproduced with less blurring or fuzziness than without the shutter. However, shutters of the prior art have been heretofore limited to shutter speeds of no faster than 1/10,000 second. While this might seem quite fast, in high speed motion studies, during athletic events, and so forth, there are numerous events which cannot be captured satisfactorily at this shutter speed.
Moreover, there has heretofore been an unsolved problem associated with fast shutter speeds. Too fast a shutter speed can effectively reduce the overall quantity of light reaching the pickup to the point where the pickup cannot properly respond. One solution is to open the lense aperture, if one is provided, to a wider f-stop--which has its own drawbacks, among them being a degradation in depth of field. Another solution is to provide other, slower shutter speeds. Thus, prior art shutters are frequently provided with a plurality of different sized matched pairs of pie-shaped apertures, each pair corresponding to a different discrete exposure time or shutter speed. With such an arrangement, however, it is not possible to continuously vary the shutter speed. One cannot, for example, select a shutter speed between two discrete speeds. Furthermore, in order to change shutter speeds using prior art systems, it is necessary to first stop the rotating shutter mechanism and lock it in place while indexing the mechanism to a new shutter speed. Varying the shutter speed while the shutter is in motion (while recording a scene, for example) is not possible with prior art systems. Hence, such systems cannot be made to readily react to transient changes in light levels, such as might be caused by a passing cloud.
Another problem with prior art rotary shutter devices stems from the use of a pie-shaped shutter opening. Video pickup tubes usually have a rectangular format, e.g. 4:3 width-to-height ratio. To evenly illuminate a rectangular pickup surface the shutter aperture must be constructed to sweep across equal areas at equal rates on both sides of the diagonal of the rectangular surface. In other words, the rectangular pickup must be oriented so that one of its vertical or horizontal centerlines coincides with the radial lines which bound the pie-shaped shutter aperture. Although there are an infinite number of locations for the rectangular pickup about a 360.degree. arc, only four of these locations (located 90.degree. apart) result in a horizontally or vertically disposed picture tube format that can be swept evenly by a pie-shaped opening. All other orientations result in a pickup surface which is skewed unnaturally to the opening. The undesirability of having a skewed format is evident when one recognizes that the camera (or viewing screen) would always have to be held or shimmed at an angle in order to render horizontal surfaces horizontal and vertical surfaces vertical.
By constraining the location of the pickup tube relative to the shutter mechanism, the overall physical design of the camera (i.e., the size, shape, and bulkiness) is appreciably affected. Such restraints, as a practical matter, result in a larger, bulkier, less aesthetically pleasing and more cumbersome to operate camera then would otherwise result were the camera designer free to place components neatly and compactly in aesthetically pleasing packages with all controls conveniently located at the fingertips.